1. Field of the Invention
The present invention relates to a CDMA wireless transmission apparatus, a CDMA wireless transmitter-receiver system and transmission power control method of the CDMA wireless transmission apparatus, more specifically to a CDMA wireless transmission apparatus which is possible to determine a maximum value of transmission power of a mobile station and/or a base station according to propagation loss between the base station and the mobile station, a CDMA wireless transmitter-receiver system and transmission power control method of the CDMA wireless transmission apparatus.
2. Description of the Related Art
FIG. 1 shows an example of construction of a mobile communication machine used in the conventional mobile communication system.
In the mobile communication system, a circular small wireless zone (cell) is formed for each base station, the mobile station communicates with the base station in the cell where the mobile station itself is present. A transmitter-receiver of a mobile station 113 is inputted with a radio-frequency signal from a reception antenna 101, which is inputted to a reception wireless section 108. The reception wireless section 108 comprises a reception amplifier 109 and a mixer 110, in which the radio-frequency signal is amplified by the reception amplifier 109 in the reception wireless section 108, and converted to a baseband signal in the mixer 110. The baseband signal is inputted to a reception baseband section 111 and converted into a reception data.
The reception data is inputted in a terminal interface section 107, where it is converted to a user data which can be interpreted by an external terminal such as telephone or video-phone, and then outputted to the external terminal. In a TPC bit analyzer 112, TPC bit inserted in the user data is analyzed. The TPC bit is a transmission power control data instructing the mobile station a transmission power to the communicating station, and the mobile station controls the transmission power according to the instruction. When the TPC bit is “00”, the transmission amplifier 103 is controlled to increase the transmission power by +1 dB, or when the TPC bit is “11”, the transmission amplifier 103 is controlled to decrease the transmission power by −1 dB.
On the other hand, when a wave is transmitted from the mobile station, the user data inputted from the external terminal is converted into a transmission data by the terminal interface 107, and the transmission data is inputted from the terminal interface 107 to a transmission baseband section 106. In the transmission baseband section 106, the transmission data is converted into a baseband signal and inputted to a transmission wireless section 103. The transmission wireless section 103 is provided with an amplifier 104 and a mixer 105, whereby the baseband signal is converted into a radio-frequency signal by the mixer 105, and, in the amplifier 104, the radio-frequency signal is amplified according to an instruction value from a TPC bit analyzer 112. Then, the radio-frequency signal is inputted from the transmission wireless section 103 to the transmission antenna 102, and a radiowave is transmitted from the transmission antenna 102.
FIG. 2 is a diagram showing an example of construction of a base station used in the conventional mobile communication system.
The signal transmitted from the antenna 102 of the mobile station 113 is inputted to the reception antenna 401 of a base station 413, and then to a reception wireless section 408. In the reception wireless section 408, the received radio-frequency signal is amplified by an amplifier 409, and converted into a baseband signal by a mixer 410. The baseband signal is inputted to a reception baseband section 411, converted into a reception data, and the converted reception data is outputted to a network through an interface 407. In a reception quality measurement section 416, reception quality is measured on the basis of information from the reception baseband section 411, and the reception quality is inputted to a TPC bit inserter 412.
When the user data Is inputted from the network, it is converted into a transmission data in the interface 407, and inputted to the TPC bit inserter 412. In the TPC bit inserter 412, when the reception quality is higher than a reference, TPC bit “11” is added, and when the quality is lower than the reference, “00” is added to the user data. The TPC bit insertion is performed at every 0.667 ms. The user data inserted with the TPC bit is inputted to the transmission baseband section 406 to be converted into a baseband signal. The baseband signal is inputted to the transmission wireless section 403. In the transmission wireless section 403, the base band signal is converted into a radio-frequency signal by the mixer 405, and the radio-frequency signal is amplified by the amplifier 404. The radio-frequency signal is inputted from the transmission wireless section 403 to the transmission antenna 402, and a wave is transmitted from the transmission antenna 402.
In the case of communication by such CDMA wireless access system, all the mobile transmission channels present in the same area use the same frequency to make communication. Therefore, when the transmission power of the transmitter is assumed as a constant value, the reception quality differs due to differences of wave propagation environment.
For example, a case is supposed in which a first mobile station and a second mobile station make communication simultaneously with the same base station. Then, wave propagation loss between the first mobile station and the base station is assumed as 60 [dB], and wave propagation loss between the second mobile station and the base station as 90 [dB]. Transmission power of the two mobile stations is +30 [dBm], reception power at the base station is +30 [dBm] for the first mobile station, and 60 [dBm] for the second mobile station. Compared to the reception power from the second mobile station, reception power from the first mobile station is higher by +30 [dB]. Therefore, since the signal from the first mobile station becomes an interference, signal from the second mobile station cannot be received.
Then, in order to avoid signal reception difficulty due to signal interference and maintain the same reception quality of all users, in the CDMA system, a technique has been known from the past to perform a closed loop high-speed transmission power control. In the high-speed transmission power control, at the base station, reception signal power to interference power ratio of reception signal is measured and compared with a reference value, and according to the comparison result, a TPC bit pattern for controlling the transmission power at the mobile station is determined.
In the high-speed transmission power control, according to communication rate, transmission power control width, and its maximum value and minimum value are determined at fixed values for all wireless channels.
However, adoption of the method in which the maximum value and minimum value of transmission power are fixed values in the conventional high-speed transmission power control has the following problems. That is, when an error occurs in signal reception for some reason, when operation of the transmitter-receiver itself becomes unstable, or when the user capacity that can be contained in the base station reaches a limit, there is the possibility that the mobile station transmits the maximum transmission power regardless of the extent of downward propagation loss. As a result thereof, there is a problem in that a deterioration occurs in the number of users that can be contained by the base station, that is, the user capacity.